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Research Report SAN 91-06 April 1990
IRRIGATION RECOMMENDATIONS FOR ESTABLISHING LANDSCAPE TREES IN FLORIDA
R. C. Beeson, Jr.
Shade trees not only increase aesthetic and monetary values of homes and
other buildings, they also lower energy cost, air pollution, and contribute to
slowing the increase in global carbon dioxide concentrations. Often after
planting, shade trees die or grow extremely slow for several years; frequently
exhibiting branch death. Such bad starts commonly distort tree symmetry,
resulting in deformed trees later in life. While there are several factors,
such as planting depth, compacted soils, buried building waste, and physical
abuse that can cause newly transplanted trees to die or undergo "transplant
shock", the most common cause is insufficient water.
On the average, 95% of the water absorbed by a tree is transpired
through the leaves, with only 5% used in the actual growth process. The water
transpired is not wasted. It allows the tree to absorb carbon dioxide from
the air, which is then converted to carbon building blocks for growth. A tree
maintained in a well-watered state is able to absorb the greatest quantity of
carbon dioxide, producing maximum growth. When water becomes limiting, so
too does the amount of the carbon building blocks, slowing growth, and
sometimes causing leaf death. If water is severely limited, branch death
occurs, followed by tree death if the situation is not corrected. Under
water-limiting conditions, trees undergo water stress. Trees subjected to
severe water stress for prolonged periods become more susceptible to insect
and disease attack, which may cause tree death. Witherspoon and Lumis (1986)
reported greatly reduced rates of root regeneration when available soil
moisture was reduced much below 100%. Therefore, there are many advantages to
maintaining a tree in a well-watered state. This is especially true for newly
planted trees, which have neither the root system nor soil mass to enable them
to survive even short periods without moderate rains.
The question is not if recently transplanted trees should be watered,
but how much and how frequently. The answer is complex and specific
recommendations would be dependent on season, tree species, soil type, and
possibly production method to name just a few. The information to make such
specific recommendations does not exist. However, based on initial transplant
experiments in well-drained soils, a conservative general recommendation is 5
gal/diameter-inch (at 6 inches above the soil line). This is limited to areas
with well-drained soils at least 7 ft deep. In more stressful sites, such as
along major roads or around large parking areas, rates of 5 to 10
gal/diameter-inch may be more appropriate. Whereas, in sites of high water
* tables or where drainage is slow, rates of 1 to 3 gal/diameter-inch may be
sufficient. Lower rates may also be sufficient during the winter months in
sub-tropical regions, but this has yet to be determined.
Irrigation frequency is as important as applying sufficient volumes.
During the first months following transplanting, root exploration into the
soil at the transplant site is minimum. Therefore, a tree is dependent on the
volume of water applied to the original football. Since root depth into the
soil does not exceed the height of the original football, once water has
drained below the original football, it is no longer available to the tree.
Thus, to minimize water stress, irrigation must be concentrated on the
football and applied frequently. Research indicates that frequencies of 2 to
3 times a day are more beneficial toward minimizing water stress and tree
establishment time than identical volumes applied once a day. Because of the
limited soil volumes from which to extract water, daily applications are
necessary for several months.
When transplanting trees, the transplant hole should be dug six inches
wider than the football. Prior to placing the tree in the hole, water should
be added and maintained in the planting hole while replacing the soil around
the football. This minimizes the possibility of air pockets around the
football and thoroughly wets the soil around the football. The hole should be
filled with the soil removed from the hole. Additions of soil amendments such
as peat moss, manure, compose, etc. have not been found to-improve transplant
success. In most cases the use of soil amendments, except in heavily
compacted urban soils, were found to increase establishment time and decrease
tree growth (Whitcomb, 1979). When placing the tree in the hole, the football
should extend one half-inch above the soil line. Placing too much soil on top
of the football will suffocate roots and lead to tree death. If the trees are
to be watered by hand, a wall of soil 3 to 4-inches high should be built
O around the edge of the planting hole. This will form a well to contain the
water where it is needed. If trees are to be irrigated with a sprinkler
system, the emitting device should not cover an area larger than planting
Once transplanted, the tree should be mulched about 3-inches deep to a
width equal to two foot plus six-inches for every diameter-inch of the tree.
For a two-inch caliper tree, the mulched area should be three foot in
diameter. The mulch will assist in maintaining adequate soil moisture and
suppress weed and grass growth that would compete with the new tree roots for
After transplanting, trees should be irrigated daily with a volume equal
to 5 gal/diameter-inch, divided into 2 or 3 application times. For trees
transplanted from containers, trees should be irrigated in the early morning
and preferably in early afternoon. Trees transplanted from fabric containers,
balled and burlap, or by tree spade, should be irrigated 3 times daily,
preferably early morning, noon, and again around 4:00 P.M. For field-produced
trees, this schedule should be continued for the first 3 months then reduced
to twice daily. These irrigation schedules should be followed the first six
months after transplanting.
After six months, trunk diameter should be re-measured six-inches above
the soil. For field-grown trees, the width of the mulched area should be
increased an additional six-inches for each diameter-inch of the trunk. Thus,
* a tree that was 2 inches and mulched 3 ft at planting will probably need a 4.5
ft mulched area. Where trees are hand-watered, the well should be expanded to
1.5 times width of original well. The volume given should be increased
according to the increase in trunk diameter and applied evenly over the area
enclosed by the new well. Where irrigation is by a sprinkler system, the
system should also be adjusted to increase the width covered to 1.5 times that
covered the first 6 months and the volume increased accordingly. Preliminary
experiments have shown that root exploration into the new site is much slower
for container-grown trees than those field-grown. Thus, no adjustment appears
necessary in the area mulched or irrigated. During the second 6 months,
irrigation frequencies should be reduced to once daily. Deviations from this
should only occur when rainfall amounts exceed 0.5 inches in a 24-hr period.
There is no direct information upon which to base recommendations for
tree irrigation beyond the first year. Studies in Florida have found root
elongation rates of 5 to 6 ft/year under conditions of sufficient irrigation
and minimum completion from weeds and grasses (Gilman, 1991). Some evidence
suggest a continuing benefit of tree irrigation for at least as many years as
the diameter-inches of the tree at transplanting. Based on the information
available, it is suggested that the mulched area be increased to 2
ft/diameter-inch at the beginning of the second year. The irrigation volume
should also be increased accordingly and the area irrigated expanded to
include the area under mulch. Irrigation frequencies of three times a week
should be sufficient during the first six months of the second year and then
to twice a week, thereafter. After the second year, further direct irrigation
may be unnecessary except during the spring months and in case of drought
during the summer for trees that were two inches or less at transplanting.
For trees of larger diameter, Watson (1985) found in the mid-west that one
year is required to regenerate the root system for every diameter-inch at
transplanting. Preliminary experiments in Florida hint at more rapid root
regeneration here. Until this is verified, it is suggested that direct
supplemental irrigation be continued twice a week until the total period of
irrigation equals one year for each diameter-inch at transplanting.
Because the volumes of water required for trees far exceeds that
required or applied for lawns, the use of a lawn irrigation system is not
sufficient to supply water during tree establishment. A separate system
should be installed for tree irrigation. Possibly the simplest and most
economical would be the installation of 3/4-inch polypropylene tubing placed
in the same trench as the PVC used for the lawn system. This could easily be
extended to the areas where trees are to be planted and connected to a
separate solenoid controlled by an additional station on a time clock.
Changes in irrigation volumes and areas could be accomplished by installation
of additional emitters and adjustments of the irrigation times. Both citrus
spray stakes and emitters are available in a wide range of volumes/hour and
coverage patterns. Where such a system is not installed, the alternative is
The recommendations given above are somewhat liberal in regards to
estimated tree water consumption. If applied as described, these rates should
minimize tree water stress and death during establishment, resulting in better
tree growth. Irrigation in excess of the recommended rates the first year
would be of no value and a waste of water. In the future, it is hoped that
these recommendations may be revised to more closely conform to actual tree
i water requirements, taking into account seasonal and species differences.
Gilman, E. F. 1991. Tree root growth and development II. Response to culture,
maintainment and planting. J. Environ. Hort. (In press)
Watson, G. 1985. Tree size affects root regeneration and top growth after
transplanting. J. Arboriculture. 11:37-40.
Whitcomb, C. E. 1979. Factors affecting the establishment of urban trees.
J. arboriculture 5:217-219.
Witherspoon, W. R. & G. P. Lumis. 1986. Root regeneration of Tilia cordata
cultivars after transplanting in response to root exposure and soil
moisture levels. J. Arboriculture. 12:165-168.